The present invention relates to a method of recording a gradation image in a thermal printer, and more particularly to a method of recording a gradation image on a thermosensitive medium with heating elements by energizing the heating elements with a combination of pulse signals each corresponding to a one-gradation image and pulse signals each corresponding to a plural-gradation image, so that the image can be recorded accurately at a high speed.
Various imaging apparatus such as conventional X-ray photographing apparatus, ultrasonic imaging apparatus, X-ray CT (Computerized Tomography) and DF (Digital Fluorography) apparatus, for example, are finding widespread use for medical applications for diagnosis or the like. In such a medical image diagnostic apparatus, ultrasonic energy, X-rays, or the like is applied to the body of a patient to produce an image of a local region of the body to be diagnosed, and the produced image is displaced as a visible image on a CRT motor, for example. A doctor or the like then diagnose the local region by observing the displayed image. Since the local region of the patient's body and, if desired, other local regions thereof can easily be observed, the diagnostic procedure can accurately and quickly be performed.
It is desirable to selectively record displayed images permanently on recording mediums to provide hard copies. A variety of printers have been proposed to make such hard copies. As an example, there is known a thermal printer which employs a light-fixable thermosensitive film that will develop a color in the pattern of an image to be recorded upon application of heat and fix the image when exposed to ultraviolet radiation.
The thermal printer has a thermal head comprising an array of heating elements oriented in a main scanning direction perpendicularly to an auxiliary scanning direction along which the light-fixable thermosensitive film is fed. An image signal supplied from the medical image diagnostic apparatus is applied to the thermal head to record a desired image on the light-fixable thermosensitive film. Then, the recorded image is fixed to the light-fixable thermosensitive film by exposure to ultraviolet radiation emitted from an ultraviolet lamp in an image fixing unit.
The light-fixable thermosensitive film develops an image density dependent on the period of time in which the film is heated by the thermal head, i.e., the amount of thermal energy applied to the film, as shown in FIG. 1 of the accompanying drawings. The thermal printer can therefore record an image of a desired density by adjusting the period of time in which the film is heated by the heating elements, based on the density vs. energy curve illustrated in FIG. 1.
One method of recording a gradation image on a light-fixable thermosensitive film with heating elements comprises energizing heating elements with as many pulse signals as the number of image gradations to heat the light-fixable thermosensitive film. According to this recording method, however; since as many pulse signals as the number of image gradations must be transferred to each of the heating elements, a long interval of time is required to transfer the pulse signal data, and the time needed to record the image is long. Between pulse signals to be transferred to each heating element, there are produced as many time zones in which the heating element remains de-energized as the number of image gradations. The temperature of the heating element is varied in those time zones, lowering the efficiency of color development on the light-fixable thermosensitive film.
There is another method of forming an image with one pulse signal for one pixel by adjusting the time duration of the pulse signal for energizing a heating element dependent on an image gradation to be reproduced. With such a method, pulse signals having pulse durations corresponding to respective image gradations have to be applied to the respective heating elements. This method also requires a long period of time to record a desired image. Because the temperature of each heating elements varies between pulse signals applied, the efficiency of color development on the light-fixable thermosensitive film is also lowered.